J. Phys. Chem. A 2008, 112, 12011–12021
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Structure of Molybdenum and Tungsten Sulfide MxSy+ Clusters: Experiment and DFT Calculations Melissa J. Patterson,† James M. Lightstone,†,§ and Michael G. White*,†,‡ Department of Chemistry, Stony Brook UniVersity, Stony Brook, New York 11974, and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: August 15, 2008; ReVised Manuscript ReceiVed: October 2, 2008
A combination of experiment and density functional theory was used to investigate the energetics of CO adsorption onto several small MxSy+ (M ) Mo, W; x/y ) 2/6, 3/7, 5/7, 6/8) clusters as a probe of their atomic and electronic structure. Experimentally, tandem mass spectrometry was used to measure the relative yields of MxSy+(CO)n cluster adducts formed by collisions between a beam of mass-selected MxSy+ cluster ions and CO molecules in a high-pressure collision cell (hexapole ion guide). The most probable MxSy+(CO)n adducts observed are those with n e x, that is, only one CO molecule bound to each metal site. The notable exception is the M5S7+ cluster, for which the n ) 6 adduct is found to have nearly the same intensity as the n ) x ) 5 adduct. Density fuctional calculations were used to search for the lowest energy structures of the bare MxSy+ clusters and to obtain their relative stability for sequential CO binding. The calculated trends in CO binding energies were then compared to the experimental adduct distributions for assigning the ground-state structures. In this way, it was possible to distinguish between two nearly isoenergetic ground-state isomers for the M2S6+ and M3S7+ clusters, as only one isomer gave a calculated CO stabilization energy trend that was consistent with the experimental data. Similar comparisons of predicted and observed CO adsorption trends also provide evidence for assigning the ground-state structures of the M5S7+ and M6S8+ clusters. The latter contain metallic cores with most of the sulfur atoms bonded along the edges or in the faces of the metal core structure. The n ) 6 and 7 adducts of M5S7+ are predicted to be more stable than the n ) x ) 5 adduct, but only the n ) 6 adduct is observed experimentally. The DFT calculations show that the n ) 7 adduct undergoes internal bond breaking whereas the n ) 6 framework is stable, albeit highly distorted. For the M6S8+ cluster, the calculations predict that the two lowest energy isomers can bind more than six CO molecules without fragmentation; however, the apparent binding energy drops significantly for adducts with n > 6. In general, the ability of these small MxSy+ clusters to bind more CO molecules than the number of metal atoms is a balance between the gain in CO adsorption energy versus the strain introduced into the cluster structure caused by CO crowding, the consequences of which can be fragmentation of the MxSy+(CO)n cluster adduct (n > x). Introduction Transition metal sulfides such as MoS2 and WS2 have been the topic of much interest because of their ability to form wellorganized cage structures on the nanoscale. Like carbon, the S-M-S layered structure of the metal sulfides has a high propensity for forming folded structures such as hollow nanotubes, nanoctahedra, and nanonions.1-6 Metal sulfide nanomaterials also display a wide range of unique catalytic, photovoltaic, and lubricant properties.7-9 For example, MS2 nanotubes have shown potential as a hydrogen storage media,10 whereas small MoS2 nanoplatelets exhibit size-dependent band-gaps and are active photo-oxidation catalysts.8 Small nanoplatelets of MoS2 (1-5 nm) are also considered to be the active species in commercial hydrodesulfurization catalysts.11 As the metal sulfide particles become very small, that is, molecular clusters, other structural motifs have been observed that are nonstoichiometric. These include the Mo6S8 moiety that contains an octahedral Mo6 * Corresponding author. E-mail:
[email protected]. † Stony Brook University. ‡ Brookhaven National Laboratory. § Present address: Naval Surface Warfare Center, Indian Head Division, Research, Development, Test and Evaluation, 4104 Evans Way Suite 102, Indian Head, MD 20640.
metallic core that is the building block of the Chevrel phase of MoS2, for example, the superconducting AMo6S8 compounds (A ) main group, transition, or lanthanide metal).12,13 Crystalline Chevrel phases and amorphous ternary molybdenum sulfides have been found to be active catalysts for methanethiol synthesis and hydrodesulfurization.14 The observation of small cluster units in the condensed phase has given rise to a number of experimental15-23 and theoretical studies24-33 of isolated metal sulfide clusters as a function of size and metal-to-sulfur ratio. In the case of molybdenum and tungsten sulfide, gas-phase cluster techniques using laser ablation and sputtering sources have been used to generate MxSy clusters as neutral,19 anion,20,31 and cation21 species with a wide range of stoichiometry. Experimental investigations are limited to probes of reactivity and electronic structure, whereas the atomic structures of the clusters are derived mainly from theoretical calculations using density functional theory (DFT). A few clusters have been observed as “magic” due to their large abundance in anion and cation cluster mass spectra and large HOMO-LUMO gaps as determined by anion photodetachment spectroscopy. One such species is the M4S6 cluster (M ) Mo, W), which is prominent in both anion20,31,34 and cation35 cluster mass distributions with measured HOMO-LUMO gaps of ∼2
10.1021/jp807318c CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
12012 J. Phys. Chem. A, Vol. 112, No. 47, 2008 eV for the neutral cluster.20 DFT calculations for the Mo4S6 cluster confirm its large HOMO-LUMO gap and predict a highly symmetric structure consisting of a Mo4 tetrahedral core with the six sulfur atoms bridge bonded along the Mo-Mo edges.20,21,31-34 The free Mo6S8 cluster with the “Chevrel” structure is also predicted to be a highly stable “magic” cluster with a relatively large HOMO-LUMO gap of 0.8-0.9 eV.33,36 Murugan et al. used DFT to show that that the structurally related Mo6S12 stoichiometric cluster has a similar HOMOLUMO gap (0.7 eV) as well as large magnetic moment (4 µB).32 A general conclusion of these DFT studies is that the small, gas-phase MxSy (M ) Mo and W) clusters energetically favor 3D structures with metal-metal bonding cores. These structures can be contrasted to the platelet structures of larger, nearstoichiometric MxSy nanoclusters, which exhibit the same S-Mo-S layered structure of bulk MS2. Nanoplatelets with sizes down to Mo10S24 have been recently observed in STM studies of self-assembled clusters on the surfaces of Au(111) and graphite.37 In the work reported here, we use the interaction of CO with mass-selected cluster beams to explore the structure of MxSy+ (M ) Mo, W) cation clusters. As demonstrated in a previous study of the M4S6+ (M ) Mo, W) magic cluster, a probe molecule such as CO or NH3 preferentially binds to the exposed metal sites on the cluster. For these gas pick-up experiments, a beam of mass-selected M4S6+ cluster cations is passed through a high-pressure collision cell containing the probe molecule (CO or NH3) diluted in a He buffer gas. Mass analysis of the resulting cluster adducts, for example, MxSy+(CO)n, provides information on the number and relative stability of the M-CO binding sites. In the case of M4S6+(CO)n, the product mass spectra showed that the cluster binds up to four (n ) 4) probe molecules with only a small amount of n ) 5 adduct observable. DFT calculations of the total CO adsorption energies for the lowest energy M4S6+ structure were consistent with the binding of only four CO molecules, beyond which the M-CO binding energy decreased dramatically. This combination of experiment and theoretical analysis provided further evidence for the highly symmetric structure of the M4S6+ cluster. As a follow up to our earlier work on the M4S6+ cluster, we present a combined experimental and computational study of the structure and CO-adduct stability for the M2S6+, M3S7+, M5S7+, and M6S8+ clusters (M ) Mo, W), which are the most prominent mass peaks in the MxSy+ cation cluster distributions. In general, we find that the CO pick-up data are very sensitive to the individual metal-CO binding energies and can be used to distinguish between geometrical isomers that are predicted by DFT to have similar total energies. For the small, sulfurrich M2S6+ and M3S7+ clusters, the experimentally favored geometries involve M-S-M bridge bonds and terminal sulfur bonds. The substoichiometric M5S7+ and M6S8+ clusters are predicted to have metallic cores consistent with trends found in earlier DFT studies. In addition to probing the relative isomer stabilities, the DFT calculations also illustrate the effects of multiple CO binding, which can induce structural changes and/ or break bonds within the cluster framework as more CO molecules are added. The latter may have implications for using such clusters deposited on supports as catalysts for heterogeneous reactions under high-pressure (high coverage) conditions. Experimental Section The experimental work was performed on a cluster beam apparatus that uses a magnetron sputtering source (Oxford, NC200U) for generating gas-phase cation clusters.38 The MxSy+
Patterson et al. clusters are formed by reactive sputtering of a metal target (Mo or W) with a mixture of 4% H2S in Ar gas in a region containing high-pressure He gas (aggregation gas). Both the sputtering and the aggregation gases are introduced into the source using variable leak valves with typical backing pressures of 7.6 Torr (4% H2S/Ar) and 22 Torr (He) for Mo-sulfide, and 7.9 Torr (4% H2S/Ar) and 24 Torr (He) for W-sulfide. The magnetron was operated at a power of 185 W. The length of the highpressure He region between the metal target and first exit aperture (cluster condensation region) for Mo-sulfide was 0.75 in. and 1.25 in. for W-sulfide. The cluster ions leaving the source enter a quadrupole ion guide, which transmits them to a quadrupole mass filter. A single cluster mass is then selected by its mass-to-charge ratio and then focused into a hexapole ion-guide, which also houses a collision cell for the reaction of the cluster with different probe molecules. Mass spectra presented in this work were taken with the hexapole ion guide kept at ground potential, so that the relative ion-molecule collision energies mirrored the kinetic energy distribution of the incoming MxSy+ ions (e2 eV in the laboratory frame). Gases of interest, in this case CO, are introduced into the collision cell using a variable leak valve with the pressure measured by a capacitance manometer (MKS). The CO sample was introduced as a mixture of 25% CO in He gas. The He gas collisionally cools the cluster adducts following the addition of CO molecules. Typical collision cell pressures for these experiments were in the range of 10-15 mTorr. The products were mass analyzed by a second quadrupole mass spectrometer and detected by a channeltron electron multiplier. All experiments were performed at room temperature. Computational Details. The theoretical calculations were performed on the MxSy+ clusters using all electron DFT with the DMol3 code. A double numerical basis set with p- and d-polarization functions, comparable in accuracy to a Gaussian 6-31G** basis set, was used with a real-space cutoff of 4.9 Å. The generalized-gradient corrected approximation (GGA), with the Becke exchange plus Lee-Yang-Parr correlation (BLYP) functional, was used. The FINE quality standard was chosen for tolerances of energy, gradient, displacement, and selfconsistent field convergence criteria of 1 × 10-5 au, 2 × 10-3 au/Å, 5 × 10-3 Å, and 1 × 10-6 au, respectively. The calculations were performed with spin state unrestricted. All of the cation isomers have a doublet state (s ) 1/2) as their final spin configuration with the exception of isomer I (C3V symmetry) of the M3S7+ cluster, which is a quartet (s ) 3/2). In the case of the highly symmetric M6S8+ cluster (Oh), where degenerate orbitals can give rise to higher spin states, the low spin configuration (s ) 1/2) is calculated to be the lowest energy. Therefore, all of the clusters have a magnetic moment of 1 µB with the exception of the M3S7+ cluster (isomer I), which is 3 µB. Relativistic effects were included in the calculations for the heavier tungsten atoms in the WxSy+ clusters.39 Density functional calculations were also used to examine the bonding of CO with the MxSy+ clusters. The total adsorption energy was defined relative to the energies of the bare cluster and the number of free CO molecules, that is,
Eads(n) ) E(cluster+nCO)- [Ecluster+ nECO]
(1)
where n is the number of CO molecules bound to the cluster, E(cluster+nCO) is the energy of the CO cluster adduct, Ecluster is the energy of the free cluster cation, and ECO is the energy of the free CO molecule. We also define the relative binding energy of a CO molecule to a cluster adduct as the change in total adsorption energies between adducts with one more adsorbate attached, that is,
Molybdenum and Tungsten Sulfide MxSy+ Clusters
∆Eads(n) ) Eads(n) -Eads(n - 1)
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(2)
Note that ∆Eads(n) represents only an apparent CO binding energy as sequential CO addition can lead to structural changes that can significantly alter the stability of the cluster adduct. Hence, it is somewhat oversimplified to attribute the total energy difference between the n and n - 1 adducts to the CO binding energy alone. In general, the total energy for binding n × CO molecules depends on the specific metal locations of each of the CO molecules on the cluster, and only the lowest energy arrangements for sequential CO addition are presented in this work. For example, we searched for the lowest energy position of the first bound CO molecule to the MxSy+ cluster ion, then searched for the lowest energy configuration among the remaining x 1 metal sites for the second CO molecule, and so on, until all of the metal sites are occupied. This approach is justified on the basis of the experimental evidence that multiple collisions with He gas are required to stabilize the internally “hot” adduct that results from CO addition. Simultaneous additions of CO are unlikely due to the large amount of internal energy (1-2 eV) that would be needed to be dissipated to prevent the CO molecules from simply boiling off the “hot” cluster adduct. Nonetheless, when the relative energies of different CO binding arrangements are small ( 6), but a more complete picture of adduct stability comes from consideration of the adduct structures. Figure 10 shows the calculated structures of the n ) 5 to n ) 7 adducts for isomer II of both Mo and W. It is clearly seen that the M5S7+ core of the n ) 6 adduct is significantly distorted from that of the bare cluster (see Figure 9) with two CO molecules bound to one of the equatorial metal atoms. The ability to accommodate two CO molecules on one metal site is attributed to the metallic nature of the four metal atoms forming the inner tetrahedron. As seen in Figure 10, the seventh CO molecule preferentially binds to the metal atom that is separated from the M4 tetrahedral core by three sulfur bridge bonds. Binding of CO at this metal site, however, leads to breaking two of the sulfur bridge bonds, with the MoS(CO)2 fragment tethered to the larger Mo4S6+(CO)5 core of the cluster through the remaining bridge bonded sulfur atom. The observation of the internally fractured n ) 7 cluster adduct will depend on its stability against further fragmentation on the time scale of our detection measurements. In this case, the dissociation of the n ) 7 adduct into two smaller fragments may be energetically driven by the high stability of the “magic cluster” fragment, Mo4S6+(CO)n.21 Dissociation would lead to cation species whose mass would fall outside the range of the intact cluster adducts in Figure 8 and were not detected. Hence, the calculated CO adsorption energetics for isomer II favor adduct formation for n ) 1-6, but larger adducts may be subject
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Figure 10. Geometry-optimized structure of the n ) 5-7 CO adducts of isomer II of the M5S7+ (M ) Mo, W) clusters as determined from DFT calculations.
to cluster fragmentation and dissociation. This overall picture for isomer II for both Mo and W is consistent with the observed product mass spectra that sharply terminate at the n ) 6 adduct. Hence, we tentatively assign the structure of the M5S7+ cluster cation to that of isomer II. M6S8+ Cluster. The product mass spectra resulting from interactions between the M6S8+ (Mo and W) clusters and CO are shown in Figure 11. In the case of Mo, the n ) 6 adduct is clearly the most favored product with only very small peaks at higher mass corresponding to CO adducts with n ) 7-9. The CO adduct distribution for the W cluster also shows a maximum at n ) 6; however, the mass peaks corresponding to the bare cluster ion and the n ) 1 CO adduct are of comparable intensity. A very small peak corresponding to the n ) 7 adduct is also observed. The fact that the product mass spectra for both Mo and W exhibit a high probability for the n ) 6 adduct is consistent with the qualitative expectation of one CO molecule bound on each of the six metal atoms of the M6S8+ cluster. Figure 12 shows the calculated lowest energy structures for the Mo6S8+ cluster (the W6S8+ cluster is structurally similar) as well as the calculated total adsorption energies for sequential CO addition to both the Mo and the W clusters. The highly symmetric structure of isomer I has been previously identified as the building block of the well-known Chevrel phase of molybdenum sulfide in AxMo6S8 or ABMo6S8 solid-state compounds.14,49 The metal atoms of isomer I are arranged to form an inner octahedron with the sulfur atoms symmetrically placed in the triangular faces (near Oh symmetry). Because the Mo6S8 moiety is known to have the “Chevrel” structure of isomer I in solid-state and solution-phase chemistry, it was expected that the calculated CO adsorption energies of isomer I would follow the experimentally observed CO adduct product distributions. In fact, the calculated total CO adsorption energies for Mo and W (see Figure 12) suggest that adducts with more than six CO molecules should be observable, whereas the experimental mass spectra show that adducts with n > 6
Patterson et al.
Figure 11. Mass spectra of products formed from collisions of the M6S8+ (M ) Mo, W) cluster with a 25% CO/He gas mixture.
Figure 12. Optimized structures of the two lowest energy isomers of the M6S8+ (M ) Mo, W) clusters. Figures adjacent to each isomer show the total adsorption energies for sequential adsorption of CO molecules to the M6S8+ clusters as determined by DFT calculations.
have very low probability. The theoretical result is perhaps not too surprising, as the M5S7+ cluster with a similar metal internal framework was also predicted to bind more than one CO per metal atom. Nonetheless, the calculations for isomer I show a significant decrease in the binding energy for placing two CO molecules onto a single metal site. Specifically, the decrease in apparent CO binding energy between the n ) 6 and n ) 7
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TABLE 4: Calculated Binding Energies, ∆Eads(n), for the Consecutive Addition of nCO Molecules to the M6S8+ Cluster (M ) Mo, W)a ∆Eads(n) Mo6S8(CO)n+
∆Eads(n) W6S8(CO)n+
n
isomer I
isomer II
isomer I
isomer II
1 2 3 4 5 6 7 8
-1.32 -1.14 -1.06 -1.04 -1.03 -1.00 -0.60 -0.60
-1.10 -1.12 -0.96 -0.95 -0.86 -1.38 -0.60 -0.60
-1.78 -1.55 -1.57 -1.45 -1.50 -1.52 -0.72 -0.54
-1.60 -1.64 -1.40 -1.41 -1.26 -2.24 -0.73 -0.54
a
All energies are given in eV.
Figure 14. Comparison of the geometry-optimized structures for CO adducts (n ) 1-4) of isomers I and II of the Mo6S8+ cluster as determined from DFT calculations.
Figure 13. Geometry-optimized structures for the n ) 6-8 CO adducts of isomer I of the M6S8+ (M ) Mo, W) clusters as determined from DFT calculations.
adducts is 40% for Mo and 53% for W (see Table 4). The latter can be qualitatively understood by inspecting the optimized structures for the n ) 6, 7, and 8 adducts shown in Figure 13. Relative to the n ) 6 adduct, it is seen that the addition of a second CO molecule to a single metal atom site introduces strain into the cluster framework as well as reorientation of the nearby CO molecules to minimize repulsive interactions. As a result, the gain in energy resulting from binding additional CO molecules is offset by the strain and CO crowding introduced into the adduct structure. The balance between these two effects will determine the overall stability of the adduct, and the current calculations for isomer I yield the correct trends for the relative adduct stabilities, but appear to overestimate the CO binding energies of the larger adducts (n > 6). In searching for other low energy isomers of M6S8+, a geometry optimization was performed in which the beginning structure was that of a bulk MS2 reduced to the proper stoichiometry. The resulting structure, isomer II (Figure 12), has a calculated total energy that is lower than isomer I for the Mo cluster (-0.22 eV) and higher than isomer I for the W cluster (+0.25 eV). The prediction of different lowest energy
structures for the Mo and W clusters is unique among the small clusters studied in this work. The structure of isomer II is similar to isomer I except that two of the face centered sulfur atoms have moved to form a Mo-S-Mo bridge bond on one edge of the base of metal octahedron and a Mo-S dangling bond with one of the metal atoms at the apex of the metal octahedron. These structural changes lower the symmetry of the cluster to Cs, but the metal atoms retain an octahedral configuration. From Figure 12 and Table 4, it is seen that the trends in calculated total CO adsorption energies for isomer II are very similar to that for isomer I for both Mo and W. Moreover, the calculated CO binding energies for the n ) 7 and n ) 8 adducts are essentially identical for both isomer I and isomer II (see Table 4). The latter can be explained by inspection of the optimized CO adduct structures as shown for the n ) 1-4 adducts of the Mo6S8+ cluster in Figure 14. For n ) 1-3, the CO adducts of isomers I and II retain their distinct structures. For n ) 4, however, the geometry optimization procedure results in isomer II rearranging to form an n ) 4 adduct of isomer I. It is also seen that the n ) 4 adduct resulting from rearrangement of isomer II has one of the CO molecules in a position different from that of the calculated lowest energy structure for the n ) 4 adduct of isomer I. As shown in Table 4, this difference in CO binding arrangement leads to somewhat different sequential CO binding energies for the n ) 5 and n ) 6 adducts. The differences in total energies for the adducts of isomer I and isomer II with n > 4, however, are relatively small (e0.02 eV), and so these different CO addition pathways are probably indistinguishable under experimental conditions. Beyond n ) 6, where all of the metal atom sites are occupied by one CO molecule, the cluster adducts formed by either isomeric pathway are structurally identical and so are the n ) 7 and n ) 8 total adsorption energies. The calculated structures and CO adsorption trends for the W6S8+ cluster are essentially the same as those for the Mo6S8+ cluster discussed above. Because of the similarities in the total energy of the CO adducts and the predicted isomer interconversion for adducts
12020 J. Phys. Chem. A, Vol. 112, No. 47, 2008 with n g 4, the experimental product distributions cannot provide unambiguous evidence for isomer I or II as being the preferred structure for the free M6S8+ cation cluster. Previous DFT studies have reported the existence of both isomers with binding energies only ∼0.1 eV apart, with the Chevrel structure (isomer I) still being the most stable.33 The present DFT calculation, therefore, gives a reasonable prediction of the relative isomer energies for the free M6S8+ cation clusters. It is also possible that both isomers I and II coexist under the conditions of our cluster source or readily interconvert depending on the internal energy of the clusters. In that case, the observed adduct product distributions would reflect the CO binding of both isomers. As shown above, however, isomer I is preferred in the presence of strongly interacting adsorbates like CO. This is consistent with the Chevrel structure being the most stable in other chemically bonded environments, for example, deposited on surfaces50,51 and in solid phases.12,13 Summary and Conclusions In this work, we have used a combination of experiment and DFT to study the CO adsorption properties of several small metal sulfide clusters, MxSy+ (M ) Mo, W; x/y ) 2/6, 3/7, 5/7, 6/8), as a way to probe the cluster’s electronic and atomic structure. Specifically, the mass distribution of MxSy+(CO)n adducts formed by collisions between the cluster cation and CO molecules is found to be very sensitive to cluster structure and provides a way to distinguish between low energy isomers predicted by theory. For the two smallest clusters, M2S6+ and M3S7+, the structural isomers differ by less than 0.1 eV, yet only the lowest energy structure yielded consecutive CO adsorption energies that were consistent with the experimental data. The predicted ground-state structures for the M2S6+ and M3S7+ clusters involve M-S-M bridge bonds as well as terminal sulfur atoms on each metal atom. Similar comparisons of predicted CO adsorption behavior to experimental CO adduct product distributions also provide evidence for assigning the ground-state structures of the M5S7+ and M6S8+ clusters. The latter contain metallic cores with most of the sulfur atoms bonded along the edges or in the faces of the metal core structure. The metallic core structure of substoichiometric clusters like M5S7+ and M6S8+ has been attributed to partially filled d-orbitals on the metal atoms, maximizing the number of sulfur binding sites at edges and faces. The latter maximize electronic interactions between the Mo-S atoms via d-s-p orbital hybridization.32,33 Experimentally, the most probable MxSy+(CO)n adducts observed are those with n e x, that is, a maximum of one CO molecule per metal site. As a general feature, the DFT calculations show that adding more than one CO molecule to a metal site causes considerable distortion of the cluster. In fact, the M2S6+ and M3S7+ (M ) Mo, W) clusters are predicted to internally fragment when n > x, which may explain the absence of such adducts in the experimental product distributions despite having a lower total energy than the n ) x adduct. Similarly, the n ) 6 and 7 adducts of M5S7+ are predicted to be more stable than the n ) x ) 5 adduct, but only the n ) 6 adduct is observed experimentally. The DFT calculations provide a possible explanation by showing that the n ) 7 adduct undergoes internal bond breaking whereas the n ) 6 framework is stable, albeit highly distorted. For the M6S8+ cluster, the calculations predict that the two lowest energy isomers can bind to more than six CO molecules without fragmentation, although the apparent binding energy drops significantly for adducts with n > 6. The ability to bind additional CO molecules is likely a
Patterson et al. reflection of the metallic nature of the M6 core of the M6S8+ cluster. Interestingly, the DFT calculations also show that the addition of four CO molecules to the M6S8+ cluster can lead to isomerization of the lower symmetry isomer to the more symmetric “Chevrel” isomer. Finally, we note that the calculated adsorption energies for the first CO molecule onto the MxSy+ cluster cations with x ) 2, 3, and 5 are very similar, with a value of ∼1 eV for the Mo clusters and ∼1.5 eV for the W clusters. Essentially, the same adsorption energies were obtained in our earlier study of CO addition to the Mo4S6+ and W4S6+ “magic” clusters (M ) Mo, W).21 The M6S8+ “Chevrel” clusters studied here show somewhat higher CO binding energies for both Mo (1.3 eV) and W (1.8 eV). These values can be compared to previous studies of CO bonding, which have focused on larger two-dimensional clusters (platelets) whose structures mimic bulk MoS2.27,28 Binding of CO only occurs at the edges of the platelets, which can be either Mo or S terminated. These 2D clusters are used as models for the active catalytic species in hydrotreating catalysts, which are composed of small nanoplatelets of nearstoichiometric MoS2. In their study of Mo28Sx (x ) 48, 60, 84), Zeng et al. obtained a first CO binding energy of 1.2 eV at 4-coordinated and ∼0.5 eV for 6-coordinated Mo edge sites.28 For the near stoichiometric Mo16Sx (x ) 29, 34, 38) clusters, the same group calculated a range of 1.1-1.3 eV for the first CO binding energies at 4-coordinated Mo edge sites (depending on exact geometry of metal site).27 Near additivity of adsorption energy is also observed for multiple CO additions on the large clusters, similar to what is predicted for the smaller Mo4S6+, Mo5S7+, and Mo6S8+ clusters in this work. Despite the large variation in structure, the first CO bonding energies are remarkably similar between these larger bulk-like clusters and the small clusters studied here, even though the Mo4S6+, Mo5S7+, and Mo6S8+ have metallic cores that are absent in bulk MoS2. These results suggest that the metal-CO interaction is highly localized with the detailed structure of the surrounding cluster playing only a minor role. Because CO is known to bind to the same metal sites that are also active for catalysis, it is likely that even the smaller clusters 3D clusters are active for promoting surface reactions. In particular, the high CO adsorption energies and high number of active sites (6) per unit volume make the Mo6S8+ “Chevrel” cluster an interesting candidate as a supported nanocatalyst. Indeed, recent DFT calculations by Seifert et al. have explored the binding of the Mo6S8 clusters on a Au(111) surface and have shown it to strongly bind to the Au surface with only minor structural changes.50,51 We are currently exploring the reactivity of the Mo6S8 cluster deposited on a Au(111) surface using mass-selected deposition as recently demonstrated for the Mo4S6/Au(111) system.35 Acknowledgment. We wish to thank Dr. Ping Liu and Dr. James Muckerman, Brookhaven National Laboratory, for helpful discussions and guidance with the DFT calculations. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-98CH10086 with the U.S. Department of Energy (Division of Chemical Sciences). References and Notes (1) Tenne, R.; Rao, C. N. R. Philos. Trans. 2004, 362, 2099. (2) Tenne, R. Chem.-Eur. J. 2002, 8, 5296. (3) Tenne, R. Colloids Surf. 2002, 208, 83. (4) Seifert, G.; Kohler, T.; Tenne, R. J. Phys. Chem. B 2002, 106, 2497. (5) Parilla, P. A.; Dillion, A. C.; Parkinson, B. A.; Jones, K. M.; Alleman, J.; Riker, G.; Ginley, D. S.; Heben, M. J. J. Phys. Chem. B 2004, 108, 6197.
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